High density serial optical data recording depends on very small marks being made with focused spots of laser light. To permit minimization of the required laser power, it is desirable to couple as much light as possible into the thin layer in which the marks are made and to avoid losses (e.g., due to reflection) from other surfaces or layers. To permit minimization of the mark size, and hence maximization of storage density, it is also desirable to use a high numerical aperture objective lens so as to form a small light spot.
A class of optical recording materials which may be used in optical data recording is thin metal films. These films are generally smooth, grainless and stable, but they reflect most of the light sent onto them, and hence require the use of a powerful laser for rapid marking. However, it is desired to reduce laser power to reduce the cost and complexity of the recording system which has led to the idea of anti-reflection coating of the metal layer.
Conventionally, an anti-reflection effect is achieved by matching the optical impedance of the opaque metal film to that of the incident radiation by forming layers of transparent dielectrics with the proper thicknesses and refractive indices between the metal film and the incident radiation. The anti-reflection layer serves to substantially increase the amount of energy absorbed from the incident radiation. Multiple layers of these dielectrics are needed to effect a significant reduction in reflectivity, but prior art design methods assume single plane wave collimated incident light and are intolerant of non-design incidence angles.
High numerical aperture microscope optics used for recording and readout means that a broad fan of plane waves, over as much as .+-.72.degree., is incident upon the anti-reflection layers. Prior art designs pick out a narrow range of these for good coupling into the film and eliminate the rest which can give a performance which is worse than having no coating at all. It is desired, therefore, to provide an anti-reflection coating which is simultaneously effective over a broad angular range of the incident laser light for both S and P polarization while maximizing the amount of incident radiation absorbed by the metal film.